HYDROGENATION OF ETHYLENE AND PROPYLENE
2543
Hydrogenation of Ethylene and Propylene over Palladium Hydride
by R. J. Rennard, Jr., and R. J. Kokes Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland (Received February 26, 1966)
21218
The rate of ethylene hydrogenation as a function of hydrogen concentration, temperature, ethylene pressure, and hydrogen pressure has been studied over palladium hydride and palladium deuteride. Similar (but less extensive) studies have been carried out with propylene. The rate of hydrogenation at -78" is found to be nearly zero order in ethylene and hydrogen pressure but first-order in the hydride concentration. Although the firstorder rate constant decreases with hydride concentration, the activity increases. There is an inverse isotope effect with deuterium, and the principal deuterated product is CzH4Dz. Analysis of the data suggests that the slow step is the addition of adsorbed hydrogen atoms to adsorbed ethylene or adsorbed ethyl radicals.
Introduction
Experimental Section
Hydrogen, deuterium, and helium were purified by passage through a charcoal trap a t -195". Ethylene and propylene (CP grade) were fractionated prior to use and checked for purity by gas chromatography. Unless otherwise noted, before any of the experiments, the palladium sample was heated for 16 hr in 200 mm of hydrogen a t 450", degassed for 0.5 hr at this temperature, and cooled in helium to -78". At all times, the palladium was protected from mercury vapor by a trap a t -78". Palladium hydride or deuteride was formed at -78" by sorption from the gas phase. Kinetic studies of the hydrogenation of ethylene were carried out on 320 mg of palladium admixed with 1.5 g of powdered quartz, both 40-60 mesh. This mixture was spread over the bottom of a 30-cc conical flask connected via a capillary stopcock to a vacuuni system. After pretreatment of the catalyst, prepara-
Palladium powder was prepared by a method similar to that used by Gillespie and An aqueous solution of palladium chloride (10%) was treated with ammonia and then with hydrochloric acid. The salt formed was reprecipitated several times and then reduced to metallic palladium by slowly heating to 500" in a stream of hydrogen. Then, the sample was flushed with helium, cooled, and washed with hot distilled water and concentrated ammonia. After this, the reduction at 500" was repeated. This procedure yielded a palladium sponge with a B E T surface area of about 0.4 m2/g.
(1) (a) G. Reinacker and E. A. Bommer, 2. Anorg. Allgem. Chem., 236, 263 (1939); (b) G. Reinacker, E. Muller, and R. Burmann. ibid., 251, 55 (1943). (2) D. A. Dowden and P. W. Reynolds, Discussions Faraday Soc., 8, 184 (1950). (3) A. Couper and D. D. Eley, ibid., 8, 172 (1950). (4) M. Kowaka, N i p p o n Kinzoku Gakkaishi, 23, 625 (1959). (5) R. J. Best and W. W. Russel, J . Am. Chem. Soc., 7 6 , 834 (1954). (6) P. H. Emmett and W. K. Hall, J . Phys. Chem., 6 2 , 817 (1958). (7) For a recent review see G. C. Bond, "Catalysis by IIetills," Academic Press Inc., New York, N. Y., 1962, pp 244-252. (8) L. J. Gillespie and F. P. Hall, J . Am. Chem. Soc., 48, 1207 (1926).
Numerous studies'-? have been made on catalytically active alloy systems in which the composition is systematically varied in an attempt to correlate activity changes to known changes in solid-state properties. In such studies, the activities of a series of different preparations are compared (with6p6or without' corrections for differences in surface areas) on the assumption that the different preparative procedures, required for different compositions, have only a trivial effect on the activity. Similar studies on the palladium hydride system offer the possibility of carrying out such comparisons on a single palladium sample. Such a study of the hydrogenation activity of palladium as a function of hydride concentration is the subject of this report.
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August 1866
R. J. RENNARD, JR.,AND R. J. KOKES
2544
tion of the hydride, and temperature adjustment, the reactant gas was admitted. From time to time a small sample of the reactant gas (1.35%) was withdrawn for chromatographic analysis on an alumina column. Preliminary tests showed that the sampling was representative.
Results Adsorption Studies. Isotherms were determined from 0 to 400" at pressures up to 500 mm. Above 100" only the o phase was present and no hysteresis was observed; below 100" the a-p transition was present and hysteresis was evident. These results were consistent with those reported by others.*-1° Even though the hysteresis is pronounced a t room temperature, however, the hydrogen could be completely removed by room temperature evacuation. (For PdH0.28 complete degassing required 1 hr; for PdHo.42 complete degassing required 3 hr.) Furthermore, even when hysteresis occurs, the pressure rapidly adjusts to a steady quasi-equilibrium value. Hydride formation with palladium was also detected at -78, -183, and even -195". At the lower temperatures, sorption is slow. At -78" with inlets of hydrogen corresponding to compositions up to PdHO.60, the half-time for sorption is of the order of 5 sec, the residual pressure is negligible, and a 30-min evacuation removes less than 0.05% of the hydrogen (from PdH0.24). Surface area measurements on a pure palladium sample yield a value of 0.81 m2, whereas those for a PdH0.42sample prepared from the same sample yielded a value of 0.79 m2. Further studies revealed that neither the standard pretreatment nor reformation of the hydride was accompanied by a detectable change in area. Figure 1 shows the pressure fall accompanying the sorption of hydrogen at -78" by PdHo.24from an equimolar mixture of hydrogen and ethylene or hydrogen and propylene. (Analysis of the gas phase shows that about 10% of the pressure fall could arise from alkane production.) I n the absence of olefin a t this hydrogen pressure (107 mm), the half-time for sorption would be about 5 sec; hence, the presence of olefin decreases the rate by about two orders of magnitude. Reaction with Hydrogen. Because of the rapid sorption of hydrogen compared to reaction it was only possible to study the reaction of a hydrogen-olefin mixture in which sorption was also occurring. To this end, the amount hydrogenation of a 50:50 hydrogen-olefin mixture over PdHo.24was compared to that of a 50:50 helium-olefin mixture. The results are summarized in Table I. It appears from these data that the rate of reaction decreases from 20 to The JOUTnaE gf Physical Chemistry
I .4
0
4
12
8
16
Time (min.)
Figure 1. Adsorption from an olefin-hydrogen mixture at -78O: open circles, 50:50 CaH&:Hz,P (total) = 214 mm; closed circles, 50:50 C2H4:H2, P (tot'al) = 214 rnm.
40% when the pressure of hydrogen decreases by four or five orders of magnitude. In other words, the reaction is essentially zero order with respect to hydrogen for reaction over palladium hydride. Table I: Hydrogenation over PdHo.24 Initial
Final
Mixture
P H ~mm ,
P H ~m, m
CaHsHz CtHe-He CzHrHz CzHa-He
107
30
. . .a
. . .a
107
34
. . .a
...
pmoles of paraffinb
3.6 3.1 24 15
On the basis of the residual pressure after sorption, this value would be of the order of 10-3 mm of Hz. Amount formed after 18 min. In this time, about 200 pmoles of hydrogen was taken up by the catalyst.
'
The effect of ethylene pressure on the reaction with two samples of palladium hydride is shown in Table 11. These results show that the reaction is nearly, but perhaps not quite, zero order in ethylene pressure. The kinetics of the reaction of ethylene with palladium hydride can be represented by the equation: In C/Co = -kt, where Co and C represent the hydrogen content of the catalyst at t = 0 and time t, respectively, and k is a pseudo-first-order rate constant that depends (9) D. M. Nace and J. 0. Aston, J. Am. Chem. SOC.,79, 3619, 3623, 3627 (1957). (10) D. P. Smith, "Hydrogen in Metals," University of Chicago Press, Chicago, Ill., 1948.
HYDROGENATION OF ETHYLENE AND PROPYLENE
2545
Table I1 : Ethylene Hydrogenation over Palladium Hydride Sample
PCZHII, mm
rmoles of ethane/hr
PdHo.396 PdHo.396 PdHo.nas PdH0.m PdH0.239
449.6 156.3 156.3 74.5 39.3
50.0 50.0 39.4 36.0 34.5
\
-+
*\
on Co. Figure 2 illustrates the fit of the kinetics to this equation for several values of Co. It is evident from these plots that this equation is valid for as much as three half-lives. Figure 3 illustrates the dependence of this rate constant on concentration by a plot (for convenience) of the half-life vs. hydrogen concentration for two different samples of palladium. The results for the two different batches of catalyst are nearly the same, and for both samples the rate constant decreases with increasing hydrogen content. It should be noted, however, that the activity actually increases with hydrogen content. This is illustrated in Figure 4 which shows a plot of the ethane formed in 1 hr os. the hydride concentration. Reaction of propylene with palladium hydride was determined a t -78" for PdHo.z3and PdHO.41. These results could also be represented by the pseudo-firstorder equation. The ratios of rate constants for ethylene to those for propylene were 8 for PdHo.Z4and 9 for PdHo.41. Similar relative rates for propylene and ethylene have been reported for conventional hydrogenation reactions." Effect of Pretreatment. Kowaka4 reported that pretreatment of palladium with hydrogen reduces the activity for ethylene hydrogenation. Details of the pretreatment are not clear; hence, we investigated the effect of several pretreatments on the activity. The results are summarized in Table 111. Various pretreatments were as follows. A. The sample was degassed a t 400", then cooled in sufficient hydrogen to form PdH0.23a t -78". B. The sample was degassed a t 400" and cooled to room temperature in sufficient hydrogen to form PdHo.23. This hydrogen was removed by evacuation a t room temperature and PdHo.23 was reformed a t -78". C. The sample was degassed a t 400" and cooled to room temperature in enough hydrogen to form PdHo.ol. It was then cooled to -78" and enough hydrogen was added to form PdHo.23.
0.4 A\
0.3
A\
I
I
60
0
I
120
I
I
240 301
180
Time (min.) Figure 2. Kinetic plots for ethylene hydrogenation: closed circles, PdHo.40; open circles, PdHo.2,; triangles, PdHo.02,.
I 1000
2
O
0
l /
0
0.12
0.24
0.36
0.40
0.6
H/Pd Figure 3. Hydrogenation half-life as a function of hydride composition: closed circles, catalyst I; open circles, catalyst 11.
Table 111: Effect of Pretreatment of pdH0.2~ on Rate Pretreatment
Standard A (see text)
B (see text) C (see text)
104k, min-1
19.3 11.7 28.5 19.3
(11) K. N. Campbell and B. K. Campbell, Chem. Rev.,31,77 (1942).
Volume 70,Number 8 Auguat 1966
R. J. RENNARD, JR.,AND R. J. KOKES
2546
1.8 t Y \ 0
-
1.6
Y
1.4 OY
I
0.10
0
I
I
0.40
0.50
I
I
0.30
0.20
1.2
H 1 Pd Figure 4. Activity us. hydride composition. The ordinate represents the amount of C2H6 formed per hour.
1.0
11: 0
0,I
I
,
I
0.2
0.3
0.4
0.
H / Pd
3'0
2.0
-
1.5
-
1.0
-
0.5
-
Y 0
0
m
c
ct
01 4.5
I
I
5.0
5.5
I
I
x to3
Figure 5. Arrhenius plot for ethylene hydrogenation: triangles, PdHo.lt; open circles, PdH0.24; closed circles, PdHo+
If we compare the activity for a standard run to that for these pretreatments, we find A reduces the activity, B increases the activity, and C has no effect. A comparable reduction in activity with pretreatment A is also found for PdHo.12, PdH0.29,and PdHo.89. E$ect of Temperature. The rate constant was determined for several hydride compositions over a temperature range from -64 to -98". These results are summarized by the Arrhenius plots in Figure 5. Values of the apparent activation energies were 8.6, 7.7, and 7.5 kcal for PdHo.11, PdH0.24, and PdHO.40, respectively . The Journal of Physical Chemistry
Figure 6. Isotope effect us. hydride composition.
Reaction with Deuterium. The reaction of ethylene or propylene with palladium deuteride followed the first-order rate law. I n general, the rate of reaction of the deuteride with olefin was greater than that for the hydride, but the effect was most pronounced a t lower temperatures and higher hydride concentrations. A systematic series of experiments with ethylene carried out alternately with the deuteride and the hydride a t -78" yielded the value of k D / k H us. hydride composition. These results are summarized in Figure 6. The ratio k D / k H is nearly unity a t very low hydride concentrations, increases abruptly near PdHo,l, and increases more slowly a t higher hydrogen concentrations to nearly 2. Near PdHoSlthe reproducibility was far worse than a t higher or lower hydrogen concentrations; possibly this occurs because the surface hydrogen concentration is often slightly above or below the gross concentration with the results that near PdHo.1 the surface can be in the high or low kD/kH region of Figure 6. The rate constant for reaction of propylene with PdDo.24 was, as with the hydride, nearly an order of magnitude less than that for ethylene. I n this case also, an inverse isotope effect was found; k D / k H for propylene was 1.9-2.0. A series of alternate deuterium and hydrogen runs was made between - 64 and - 98" for the single composition PdHo.zror PdDo.zr. These results are shown in Figure 7 and indicate that k D / k H changes from 1.5 a t -64" to 2.3 at -98". The activation energy for the deuterideis about 1 kcal less than that for the hydride (7.9 kcal) .
HYDROGENATION OF ETHYLENE AND PROPYLENE
1.6
-
1.2
-
s 0,
0
+
t
2547
-
0,s
t\I
0*4
0
4.5
' e
5.0
I TX x
5.5
0
1
2
3
4
5
6
No. of D-otoms / Molecule
io3
Figure 7. Arrhenius plot for reduction by hydride and deuteride: open circles, PdDa.,c; closed circles, PdHo.la.
For the reaction with deuterium, it was possible to estimate the deuterium content of the product ethane. The fragmentation pattern for deuterated ethanes was assumed to be that of the nondeuterated ethane (at 25 v) with appropriate statistical corrections and a relative probability of 1.2 and 0.8 for cleavage of C-D and C-H bands, respectively.12 The results at -78", together with those reported by Bond and Wells13 for supported palladium a t -36O, are summarized by the smoothed curves in Figure 8. Clearly there is much less mixing of deuterium for ethane production by palladium deuteride.
Discussion Palladium hydride is a two-phase system; .both phases, a and p, are cubic close packed with respect to palladium atom^.'^^'^ At room temperature, the CY phase alone is present for hydrogen concentrations below PdHo,05and the /3 phase alone is present for concentrations above PdHO.jj. At intermediate concentrations both phases, as judged by X-rays, coexist. At the temperatures of interest to us, the hydrogen atoms are inobilelj and occupy the octahedral holes in the close-packed lattice.16 The hydrogen-palladium system is not a typical two-phase system. In the two-phase region the hydrogen fugacity increases with hydrogen content." (Such behavior can be accounted for in part by analysis of stress eff ects.Is) Furthermore, physical properties such as magnetic susceptibility do not reflect the phase transitions. The molar magnetic susceptibility
Figure 8. Isotopic distribution in product ethane: open circles, PdDo.2aplus ethylene a t -78"; closed circles, hydrogenation over supported palladium a t -36O.13
decreases linearly as the hydrogen concentration is increased and reaches zero near PdHo.6.19 This can be rationalized by the assumption that each hydrogen atom donates one electron to the existing holes in the d band of palladium. Similarly, the relative resistivity increases linearly with hydrogen content up to PdH0.v6. We are interested primarily in the relation of activity to electronic structure, and the electronic structure, as judged by susceptibility and resistivity, depends on the hydrogen content, not on what phases are present. Accordingly, we shall focus our attention on the variation of activity with hydrogen content alone. The very rapid uptake of hydrogen at - 78" suggests a rapid transfer of hydrogen between the surface and bulk palladium. Zero-order dependence on hydrogen pressure for olefin hydrogenation is in line with this; for, then, we would expect the surface concentration of hydrogen to be controlled by the bulk concentration and to be independent of the gas phase concentration. The nearly zero-order dependence of hydrogenation (12) D.0.Schissler, S. 0. Thompson, and J. Turkevich, Discussions Faraday Soc., 10, 46 (1951). (13) G . C. Bond and P. B. Wells, Advan. Catalysis, 15, 91 (1964). (14) S. D. Axelrod and A. C. Makrides, J. Phys. Chem., 68, 2154 (1964). (15) R.E.Norberg, Phys. Rev., 86,745 (1952). (16) T.R. P.Gibbs, P r o p . Inorg. Chem., 3, 422 (1962). (17) D.H.Everett and D. Norden, Proc. Roy. 8oc. (London), A254, 341 (1960). (18) N. A. Scholtus and W. K. Hall, J. Chem. Phys., 39, 868 (1963). (19) C. Kittel, "Solid State Physics," John Wiley and Sons, Inc., New York, N. Y.,1958,p 334.
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R. J. RENNARD, JR.,AND R. J. KOKES
2548
rate on olefin pressure suggests the surface is nearly covered with olefin. The reduction of hydrogen sorption rate by a factor of to when olefin is present supports this view. If the above be true, the rate-controlling step is likely to be one of the following C Z H
+ CZH4 I_ C2H5 H + CzHs CzHs(g) 2CzH5 CzH4 + CZ&(g) H
+
+
I n the above sequence, C represents the concentration, C2Hs(g) represents gaseous ethane, and all other species are assumed to be attached to the surface. Reactions 2 and 3 are usually assumed to occur in olefin hydrogenation, and reaction 4 has been considered by BondeZ0 I n any event, if the steadystate approximation is applied to C2H6,we obtain
(It is, of course, conceivable that the rate of hydrogenation is controlled by diffusion of hydrogen from the bulk to the surface. This can be ruled out on three counts: (a) The kinetics are not consistent with diffusion. (b) The order of magnitude difference in rate for ethylene and propylene is not consistent with a rate controlled by diffusion. (c) The inverse isotope effect for the reaction is not consistent with the normal isotope effect found for diffusion.)21 The marked difference in rate for ethylene and propylene rules out reaction 1 as the rate-controlling step. The lack of isotopic mixing shows that the reverse of reaction 2 can, in the first approximation, be neglected. If we then make the assumption that the reverse reaction (1) is much more rapid than (2), we can write
On integration, with CzH4 constant, this yields the observed form In C/C,
=
-kt
where the constant k is a composite quantity given by
The observed kinetic isotope effect is qualitatively consistent with the conclusion that the slow step is the rate of addition of a surface hydrogen atom to adsorbed olefin. Figure 9 shows on the left an energy The J O U T T of ~Phyaical Chemistry
Figure 9. Relative energies of PdH and PdD (see text).
diagram for gaseous hydrogen, deuterium, palladium hydride, and deuteride. I n order to construct this diagram, it was assumed that differences in the heat of formationg of palladium deuteride and palladium hydride, 8.6 and 9.6 kcal, respectively, stem primarily from zero-point energy effects. From these data and the zero-point energy of hydrogen vs. deuterium, we find that the difference in zero-point energies for PdH and PdD is about 0.8 kcal. If, consistent with the kinetic analysis, it is assumed that the slow step is the addition of a surface hydrogen to a carbon atom, the activated complex will have a nearly normal carbon-hydrogen band. Data for CC13D vs. CCl3HZ2 reveal the zero-point energy difference for this C-D us. C-H bond is 2.1 kcal. With the same figure adopted as the maximum zero-point energy difference for the complex, we obtain the energy diagram on the right of Figure 9. If we accept the foregoing qualitative analyses as correct, the activation energy for reaction with palladium deuteride is ut most 1.3 kea1 less than that for palladium hydride. Thus, hydrogenation would be expected to show the observed inverse isotope; the agreement with the observed difference in activation energies (1.0 kcal) is regarded as fortuitous. (We have no convincing explanation for the falloff in isotope effect a t low concentrations of hydrogen. Data are not available for low hydride concentrations which would permit construction of a parallel t o Figure 9. We believe, however, that this change in isotope effect may be indicative of a change in mechanism, perhaps associated with the pure a phase.) The first-order rate constant depends only on the (20) G. C. Bond, Trans. Faraday SOC.,52, 1235 (1956). (21) W. Jost, “Diffusion,” Academic Press Inc., New York, N. T., 1952, p 308. (22) G. Herzberg, “hlolecular Spectra and Molecular Structure, 11,” D. Van Nostrand and Co., Inc., New Tork, N. Y.,1945,p 316.
HYDROGENATION OF ETHYLENE AND PROPYLENE
initial hydrogen concentration and remains constant as the hydrogen content is reduced by reaction with ethylene. Michel and G a l l i ~ oreported t~~ that although sorption of hydrogen reduces the magnetic susceptibility, removal of sorbed hydrogen by reaction a t low temperature does not restore the initial magnetic susceptibility. Implications of this observation have been recently discussed by Cribbs.I6 If this observation is correct, it would mean that the electronic properties of the catalyst are governed wholly by the initial hydrogen content. Thus, provided kl/k-l and (C2H4) do not depend on the initial hydrogen content, k2 decreases as the holes in the d band are filled. Regardless of the validity of the analysis in the preceding paragraph, however, the following conclusion can be stated without equivocation. The activity increases as the holes in the d band are filled; the firstorder rate constant decreases as the holes in the d band are filled. This raises questions about correla-
2549
tions attempted solely on the basis of activity without kinetic analyses. The analysis of the data obtained with the standard pretreatment yielded a reasonable but admittedly tentative interpretation. Effects of varying this pretreatment are too complex for detailed interpretation. It is, however, worth noting that cooling in hydrogen poisoned the catalyst as has been observed for nickeP4 and also palladium.26 Perhaps these effects are due to changes in surface structure noted by Germer.26
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. (23) A. Michel and M. Gallisot, C m p t . Rend., 208, 434 (1939). (24) W.K.Hall and P. H. Emmett, J.Phys. Chem., 63, 1102 (1959). (25) A. Couper and D. D. Eley, Discussions Faraday SOC.,8 , 172 (1950). (26) L. H.Germer, Advan. Catalysis, 13, 191 (1962).
Volume 70, Number 8 Auguet 1966
